The ZXing project provides a standalone barcode reader application which — via Android's intent mechanism — can be called by other applications who wish to integrate barcode scanning.
The easiest way to do this is to call the ZXing SCAN Intent from your application, like this:
public Button.OnClickListener mScan = new Button.OnClickListener() {
public void onClick(View v) {
Intent intent = new Intent("com.google.zxing.client.android.SCAN");
intent.putExtra("SCAN_MODE", "QR_CODE_MODE");
startActivityForResult(intent, 0);
}
};
public void onActivityResult(int requestCode, int resultCode, Intent intent) {
if (requestCode == 0) {
if (resultCode == RESULT_OK) {
String contents = intent.getStringExtra("SCAN_RESULT");
String format = intent.getStringExtra("SCAN_RESULT_FORMAT");
// Handle successful scan
} else if (resultCode == RESULT_CANCELED) {
// Handle cancel
}
}
}
Pressing the button linked to mScan would launch directly into the ZXing barcode scanner screen (or crash if ZXing isn't installed). Once a barcode has been recognised, you'll receive the result in your Activity, here in the contents variable.
To avoid the crashing and simplify things for you, ZXing have provided a utility class which you could integrate into your application to make the installation of ZXing smoother, by redirecting the user to the Android Market if they don't have it installed already.
Finally, if you want to integrate barcode scanning directly into your application without relying on having the separate ZXing application installed, well then it's an open source project and you can do so! :)
Edit: Somebody edited this guide into this answer (it sounds a bit odd, I can't vouch as to its accuracy, and I'm not sure why they're using Eclipse in 2015):
Step by step to setup zxing 3.2.1 in eclipse